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Old Feb 13, 2013, 09:59 PM
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derp express's Avatar
Joined Dec 2012
160 Posts
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L/D ratio - wings vs conventional planes

Hey gents,

Another noob question here. I've been looking at drag polars for a lot of different airfoils and it seems that many decently thin ones that operate in the range of 100,000 Re to 150,000 Re or so seem to have max L/D ratios of around 50 and above. Granted, this is just the airfoil - and also doesn't factor in induced drag - that I'm aware of. However, unless we're talking about high performance gliders, it seems to be rare that any airframe has that sort of max L/D. Props, fuselages, tails, and a plethora of other drag-creating odds and ends attached to the airplane seem to really bring that number down significantly from the max L/D of the wing itself. To compound the drag issue, it seems that little model airplanes suffer from higher drag coefficients because of their low reynolds numbers. However, it seems like one possible way to help this issue would be to use a pure flying wing like a zagi / zephyr. So I've got two questions:

1.) What, at least in your personal opinion, would you consider a poor, medium, and good L/D ratio for a powered model airplane?

2.) Is there enough benefit from a pure flying wing like a zagi, or a plank to outweigh the stability issues they pose?

I've used spreadsheets to tinker with different variables like aspect ratio, span efficiency, zero lift drag coefficient, etc to try to get rough ideas of L/D for my models, but I'd really like to know what you guys consider to be the good, the bad and the ugly.


Regards,

-DE
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Old Feb 13, 2013, 10:08 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,350 Posts
Don't confuse two dimensional "infinite span" airfoil data with real wings. Once you build a model you really need to go some to get up to 20:1.
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Old Feb 14, 2013, 09:36 AM
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richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
6,863 Posts
Real world - as noted - a different ball game
I use a spreadsheet to collect shavings - First - the structure of a wing is number one concern- not some finite shape .
find out what weight and speed brackets you will be using - THEN look at a shape which will allow you to build a wing strong enough and light enough to do the job
most usable models fall somewhere in the 10 -1 down to 2 -1 aspect ratio- the speed and maneuveability desired determine this
lastly - futz with the airfoil.
High performance gliders?
depends on your ideas of performance
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Old Feb 14, 2013, 11:59 AM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
3,598 Posts
The highest L/D airplanes flying today are not flying wings. Full-size conventional aft-tailed sailplanes have higher best L/D, better minimum sink rate, better L/D at high speeds, better stability and handling characteristics, and better structural weight efficiency than the best flying wing gliders.

The best performing RC sailplanes are also conventional aft-tailed designs, despite many attempts to match the performance with the supposedly "lower drag flying wing". It turns out that the compromises that must be made in lift distribution and trim drag for stability always reduce the performance of flying wings below that of a conventional layout - mostly well below.

A good airfoil will have an infinite span best L/D of over 150:1 at full size Re. Full-size airplanes have best L/Ds of 10:1, to 60:1. Similar models will be in the range of 3:1 to maybe 28:1, because of Re effects.

What is your goal for this airplane? Why are you concerned about best L/D, when power RC models are rarely flown anywhere near that speed? Best L/D is important for going the farthest on the minimum energy. That can be very important for some full-size airplane designs, but rarely is for power RC airplanes. Maximum L/D at the design speed is a very different design goal than maximum L/D period.

Kevin
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Old Feb 14, 2013, 04:38 PM
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United States, UT, Salt Lake City
Joined Oct 2007
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Real problem with the model is pretty simple:
We have no effective method of obtaining accurate feedback
Even with all the recent Telemetry -it's a guess
best bet is a simple setup which can be tweaked in a repeatable recordable manner. Flying wings are just very short aircraft - and as such have some pretty nasty characteristics if balanced or setup incorrectly. Why do you think so few full sized craft are NOT flying wings? except for hang gliders --
I have to guess at best trim for my glider-depending on the day---
I have the very latest transmitters with umteen adjustments /mixes/ trimmers etc. etc., - all worthless till I get into the ballpark -base on simply watching and tweaking.
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